1. Field of the Invention
The present invention relates to a method and apparatus for laser heat treatment for a metal wire, especially for a steel wire used for automotive tires and so forth.
2. Description of the Related Art
Due to higher-performance of automotive engines and the subsequent increase in speed of automotive vehicles, steel wires used for automotive tires and so forth, are required to have higher tensile strength. In order to satisfy contemporary requirements, a tensile strength of at least 280 Kgf/mm.sup.2 in a steel wire of 0.3 mm diameter is necessary and, at present, steel wire having a tensile strength of 340 Kgf/mm.sup.2 is available. Furthermore, there is some steel wire having a tensile strength of 360 Kgf/mm.sup.2. However, steel wire with high tensile strength does not have fatigue strength sufficient for practical use.
It is known that fatigue strength can be improved by annealing the surface of the steel wire. Therefore, there are proposals for surface enhancement by performing heat treatment using a laser beam.
Surface treatment using a laser beam has advantages in that the heat treatment can be done on the surface of the articles to be treated and can be performed in a short period. On the other hand, however, it is difficult to establish a uniform energy distribution for a relatively wide area because of the high directionality and convergence of the laser beam. For uniform distribution of the laser beam energy, a converging optical system, such as a defocused beam type, a split mirror line beam type, or a beam scanner type, are practically employed. However, since the energy density to be irradiated on the article to be processed is lower than that used for laser welding or laser cutting and so forth and the reflectivity of the laser beam at the surface of the processed article is high, there is a necessity for the application of a beam absorbing material. Also, since the reflectivity increases according to a reduction of incident angle, adjustment for incident angle of the laser beam becomes necessary.
On the other hand, Japanese Unexamined Patent Publication (Kokai) 61-170521 discloses irradiation of a laser beam in a direction approximately perpendicular to the longitudinal axis of the processed article or in the axial direction in case of heat treatment of elongated articles, such as steel wires or round bars and so forth, employing the laser beam. However, in the former case, i.e. irradiating the laser beam in approximately a transverse direction, absorption of the laser beam becomes non-uniform in the circumferential direction of the processed article to lower the energy efficiency. Improvement in the energy efficiency can be expected by applying a laser absorbing material on the surface of the processed article. However, difficulty arises in the application of the laser absorbing material. In particular, in the case of an application of laser absorbing material in an on-line process in the production line clearly requires increasing the process steps and thus causes an increase in production costs. On the other hand, in the latter case, due to a small incident angle, reflectivity of the laser beam increases to significantly lower the energy utilization efficiency.
For improving such problems, the inventions have proposed, in Japanese Unexamined Patent Publications 63-262414 and 2-101112 which have been commonly assigned to the assignee of the present invention, methods for the heat treatment of elongated articles, in which a cone shaped mirror with a polygon or circular cross section is employed for irradiating the laser beam irradiated along the center axis thereof and heat treatment for the processed article is fed though the cone shaped mirror by multiple reflection. In the proposed process, the laser beam irradiated along the center axis of the mirror causes multiple reflection on the mirror surface and converges toward the peak of the mirror enabling a heat treatment with enhanced energy efficiency. The disclosure of the above-identified Japanese Unexamined Patent Publications 63-262414 and 2-101112 are herein incorporated by reference.